Flexible ferrite-particle magnets

ABSTRACT

Process for making barium ferrite particles which are especially adapted to mechanical orientation in admixture with a workable nonmagnetic matrix material to provide flexible magnets of extraordinarily high magnetic values. Starting with acicular alpha-Fe2O3 particles, BaCO3, a fluxing agent such as NaF and a lead compound such as PbO, magnetic particles are obtained which provide, when oriented in a rubber matrix, permanent magnet material having a maximum energy product of at least 1.4 X 106 gauss-oersteds.

United States Patet Riedl et a1. Sept. 2, 1975 [54] EXIBLEFERRITE-PARTICLE MAGNETS 2,999,275 9/1961 Blume. Jr 264/DIG. 5s3,093,589 6 1963 DO t 1.. 264 DIG. 5s

[75] lnvemors: Kenneth Ried'; Karl 3,278,440 10/1966 252/6263 both ofPaul 3,387,918 6/1968 Moore Ct 211. 264/D1G. 5s

[ Assigneer Minnesota Mining and FOREIGN PATENTS OR APPLICATIONSWmufacturmg Company Paul 284,335 4/1966 Australia 252/6263 Mmn.

[22] Filed: Feb. 24, 1972 Primary Examiner-Donald .1. Arnold pp NO:229,200 Attorney, Agent, or firm-Alexander, Sell, Steldt &

Related US. Application Data Continuation-impart of Ser. No. 63,299,Aug. 12, 1970, abandoned.

References Cited UNITED STATES PATENTS 1/1957 Crowley 252/625 8/1958Peterman 264/D1G. 58

DeLaHunt [5 7] ABSTRACT Process for making barium ferrite particleswhich are especially adapted to mechanical orientation in admixture witha workable nonmagnetic matrix material to provide flexible magnets ofextraordinarily high magnetic values. Starting with acicular a1pha-Fe Oparticles, BaCO a fluxing agent such as NaF and a lead compound such asPbO, magnetic particles are obtained which provide, when oriented in arubber matrix, permanent magnet material having a maximum energy productof at least 1.4 X 10 gaussoersteds.

10 Claims, N0 Drawings FLEXIBLE FERRITE-PARTICLE MAGNETS REFERENCE TORELATED APPLICATION This application is a continuationin-part ofapplicants copending application Ser. No. 63,299, filed Aug. 12, 1970and now abandoned.

FIELD OF THE INVENTION The present invention relates to flexiblepermanent magnets consisting of aligned ferrite particles in anonmagnetic matrix of flexible resin or rubber.

BACKGROUND OF THE INVENTION Flexible permanent magnets have beenmanufactured for many years by mixing ferrite particles with a flexibleresin or rubber. The most versatile such magnets are made by the processof U.S. Pat. No. 2,999,275 (Blume). As there disclosed, plate-shapeddomain-size particles of barium ferrite which have a preferred directionof magnetization normal to the two parallel surfaces may be mixed withrubber on a rubber mill in amounts up to about 70% of the total volumeof the mixture. Mechanical forces incident to the rolling progressivelycause the ferrite platelets to become parallel to the surface of therubber sheet. The resulting thin sheets may be cured and magnetized assuch or laminated to a desired thickness. Magnets punched out from thesheets or laminates may have a residual induction of 2 l gauss, acoercivity H of 1200 oersteds and a maximum energy product of 0.9 Xgauss-oersteds in the preferred direction of magnetization. Magnetspresently being produced commercially in this manner have somewhathigher values, perhaps in part due to treatment of the ferrite with anaqueous acid solution as described in U.S. Pat. No. 3,387,918 (Moore etal.). Such magnets typically have a residual induction of 2150 gaus s, acoercivity H 1750 oersteds, an intrinsic coercivity H of 3000 oerstedsand a maximum energy product BH of about 1.1 X 10 gausseoersteds.

The Blume patent also teaches that the ferrite platelets may beincorporated into a thermoplastic resin of the polyvinyl chloride typeand extruded through a nar row orifice into elongated shapes. Theshearing forces during the extrusion process cause the ferrite plateletsto become oriented or aligned in a mechanical way. If desired, theextruded material is squeezed between rolls while being subjected to amagnetic field such that the lines of force are perpendicular to the topsurface of the material as disclosed in U.S Pat. No. 3,312,763(Peccerill et al.).

Magnets made by the process of the Blume patent have achievedconsiderable commercial success. To a large extent this is due to theirflexibility and toughness and to their amenability to be shaped and cutto precise dimensions. However, some important potential uses forflexible magnets call for higher magnetic values than have beenattainable prior to the present invention.

THE PRESENT INVENTION The present invention concerns barium ferriteparti' cles which are especially adapted to mechanical orientation inadmixture with a workable nonmagnetic matrix material. The novel bariumferrite particles are prepared from a mixture of:

where x is the weight percent of fluxing agent but not more than about 2weight percent. Regardless of the amount of fluxing agent, flexiblemagnets of highest maximum energy products are attained where the amountof the lead compound exceeds weight percent and is less than 1% weightpercent.

The straightforward procedure of:

1. homogeneously mixing the above materials,

2. calcining the mixture at 850-l 100C, and

3. treating the resultant ferrite with aqueous acid solution to removeundesirable reaction products and any unreacted material providesparticles which are especially adapted for the manufacture of flexiblepermanent magnets by the process of the above-discussed Blume patent,that is, by the further steps of:

4. mixing the elongated platelets with a workable rubber orthermoplastic matrix material to provide a mixture comprising about55-70% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets bymechanical shearing forces, and

6. then vulcanizing the rubber or cooling the thermoplastic matrixmaterial to lock the ferrite platelets in place. By employing thisprocess to make flexible magnets, maximum energy products exceeding 1.4X 10 gaussoersteds have been consistently attained along with other highmagnetic values, e.g., B at least 2500 gauss, H above 2000 oersteds andH above 3000 oersteds. If a sheet of the magnet material is subjected toa magnetic field extending perpendicular to the surface of the sheetwhile the matrix is in a semi-fluid state, a small degree of furtherenhancement of magnetic values is realized, as evidenced by maximumenergy products on the order of 1.5 to 1.6 X 10 gauss-oersteds.

When magnetic orientation is combined with mechanical orientation, themagnet material should be subjected to the magnetic field at a time whenthe matrix material is semifluid and maintained until the matrixmaterial has set to a consistency locking the ferrite particles inplace. When the matrix material is a thermoplastic resin, it isconvenient to apply the magnetic Maximum energy products above 1.5 X 10gaussoersteds can be attained without the need for magnetic orientationby employing at least weight percent of fluxing agent plus at leastweight percent of the lead compound. The highest energy products arerealized at about to weight percent of fluxing agent. If the leadcompound is omitted and the fluxing agent con tent exceeds 5 weightpercent, maximum energy products of -l.4 X 10 gauss-oers'teds can stillbe attained. Evenlthough the use of relatively large percentages offluxing agent involve large weight losses, e.g.,even 30% or higher, thegreater energy products that can be attained create new and valuableuses for flexible magnets.

The achievement of these results requires starting materials which arereasonably free from impurities. For example, some commercial sources ofalpha-F0 0 contain undesirably high sulfur content which can readily bereduced to satisfactory levels by roasting the particles at 700Cuntilthe sulfur content is 0.3 weight percent or less.

EXAMPLE 1. A

pounds of the dry mixture were placed in agas-fired rotary calciner at950C for l hour to. provide leadmodified barium ferrite particles.

-Forty-eight hundred ml of 5% l-lCl solution were heated to 95C in aglass flask, and 1440 grams of the barium ferrite particles were added.After stirring for vr'n inutes,--with the temperature at 8793C, the

acidic solution 'was decanted and the ferrite was rinsed 5;.timeszinwarm water, once in acetone, and then was dried in an air-circulatingoven at 150C. The product ferrite particles were elongated platelets ofgenerally submicron $17.0. 7

. 1 3-1 5% weight loss occurred during the acid treat- ,mentgwhich isconsideredsatisfactorily small in view of the enhanced resultsattributable to the acid treatment. Although. .a moresevere acidtreatment might further improve magneticwalues, a weight loss above,about is considered economically ,unjustifiable in the practice. of thisexample.

, To. produce flexible magnets,

the following composition was prepared:

' Grams The acid-treated lead=modified barium ferrite v 3250Butadicne-acrylonitrile copolymer. I medium-level-nitrile (ChemigumN-608") 260 Zinc stearate I 7 Sulfur 2.5

Benzothiazyl disulfide accelerator (Altax") The rubber and zinc stearate.were mixed in a Banbury.

mixer with gradual addition of the ferritesThe mixture .was pulverizedand placed in a dry blender with the sul fur and accelerator. After 5minutes this was sheeted between cold rolls (0.015 inch roll spacing,roll ratio 1:1). Thesheet was repeatedly doubled over and put throughthe. rolls at gradually increased roll spacing until a laminate of 32layers was obtained having a thickness of about 0.140 inch. Thislaminate was put through the rolls again at gradually reduced rollspacing until its thickness was reduced to 0.125 inch.

A day later this was again passed through the rolls repeatedly atgradually reduced roll spacing to a thickness of 0.050 inch and thenrepeatedly doubled over and put through the rolls'at gradually increasedroll spacing until a laminated sheet of 32 layers of laminate wasobtained ata thickness of 0.140 inch, which was brought down to afinished thickness of 0.125 inch by additional passes. The finallaminated sheet was vulca nized in an air-circulating oven at C for onehour.

Two /z-inch diameter plugs were punched from the cured sheet and stackedfor testing, with the following results (averaged from three identicalpreparations):

2520 gauss H i v 2100 oersteds H,.,- r 3310 oersteds nmJ' 1.45 X 10"gauss-oersteds.

v EXAMPLE 2 The following composition was prepared:

The rubber with the stearic acid and wax was banded on a cold rubbermill(two mill rolls 3 inches in diameter and '8 inches in length, rollratio 1:1), initially at a roll spacing of about 0.015 inch. The ferritewas added to the banding rubber while the roll spacing was graduallyincreased to accommodate the increased bulk. When all the ferrite'wasincorporated, the sulfur and accelerator were added. The sheet, which atthis point had a-thickness of about 0.0700.075 inch, was repeatedlydoubled over and put through the rolls until a laminate of 32 layers wasobtained at a thickness of about 0.090. inch, and this was again putthrough the rolls without folding to bring its thickness down to about0.082 .inch. 1

A piece of, the sheet was placed in an aluminum box wrapped in a heatingjacket and subjected to a 13.5 kilogauss field perpendicular to thesurface of the sheet. The direction of the field was reversed six timesand then maintained constant while the temperature was brought up to150C over about 40 minutes. After additional 30 minutes at 150C, thesheet was allowed to cool to 50C over a period of about 45 minutes, atwhich time the field was removed and the sheet was taken from the box.Three /2-inchdiameter plugs were punched from the cured sheet andstacked for testing, with the following results:

2570 gauss r 2260 oersteds 0! 3900 oersteds BH 1 .55 X 10gauss-oersteds.

the Blume patent,the fcrri te par ticles should comprise at least 55%and preferablymorc than 60% of the magnet material 'by volume to providedesirably high ,magnetic values. Above about 65% the resultant magnetsmay have less integrity and flexibility than desired. but adequatephysical properties for many usesvha v e been attained at 70% ferrite byvolume. In any-event, a flexible magnet of one-eighth inch thicknessought to withstand bending over a SinCh'mandrel without breaking.

EXAMPLE "3 y Charged to a dry blender were 1761.4 grams of the sameacicular alpha1-Fc O -used in Example*l,,385r8 grams of BaCO and186.6:grams of NaF: 500'grams of the dry blended raw material mix wereplaced in an alumina sagger which was held in a globar furnace at lO50Cfor80 minutes. The produetbarium ferrite particles were acid treated inthe same manner as in Example'l. A-2O percent, weight lossoccur'redduring the acid treatment. The acid-treated particles wereelongated plateletsrof generally submicrorrsize;

To produce flexible magnets, the following composition was prepared:

The rubber, stearic acid, sulfur andpolyether'plastieizer were mixed ina Banbury mixer with gradual addi tion of the ferrite. The mixturepulverized and placed in a dry blender with the zinc oxide and the twoaccelerators, After a 5-minute blend, this was stieeted and laminatedbetween cold rolls and then vulcanized by the same procedurea s inExample li The cured sheet, had the following properties;

2475 gauss 2260 oersteds m 4300 oersteds I 1.47 X gauss-ocrsteds.

mIIcc EXAMPLE 4 1761.4 grams of acicular alpha-Fe Q, particles (5 l6-M)were combined with 385.8 grams of BaCQ 187.8 grams NaF and 23.6 gramsPbO in a dry blender. 500 grams of the dry blended raw material mix wereplaced in an alumina sagger which was introduced into a globar furnaceat 1 100C and held at this temperature for 85 minutes to formlead-modified barium ferrite particles. The particles were acid-treatedas in Example 1 except using an 8.8 percent HCl solution. Weight losswas 27%.

To provide flexible magnets, the acid-treated, leadmodified bariumferrite particles were processed in the same way and with the samecomposition as in Example 3 except that the amount of ferrite particleswas increased to 3600 grams so that the resultant magnet sheet was 69.5volume percent ferrite (as compared to 65% in Example 3). Also, thepolyether plasticizer was omitted. The magnets evidenced:

,. i 2690 gauss i 2425 oerstcds 3625 oersteds 1.72 X It)"gauss-oersteds.

r'l HIH'H'. These examples illustrate the effectiveness of NaF as thefluxing agent, Other fluxing agents are listed in the tables at pages 9and 10 of Australian Pat. No. 284,335.

We claim: v i l i v 1. Process comprising the steps of:

l. homogeneously mixing 5 v a. acicular alpha-Fe O ,of high surfacearea; i b. BaCQ, or equivalent source of barium oxide in amount toprovide upon reaction with the alpha- Fe O a ferrite of the generalizedformula BaFe j 2 1; i p c. about 1- an y d. where the amount of fluxingagent in weight percent .r is less than 6, at least 1 l2 weight percentof NaFfluxin g agent,

weight percent of a lead compound up to about 2 weight percent of themixture, 2. calcining the mixture at about 850l C, and

:3. treating the ferrite with aqueous acid solution to removeundesirable reaction products and any un' reacted material to provideelongated platelets which areespecially adapted .to mechanical orien itation in admixture with a workable nonmagnetic matrix material, I 7

2. Pr0cess for makingbarium ferrite particles which areespeciallyadapted to mechanical orientation in admixture with a workablenonmagnetic matrix material, which process comprises the steps of:

' l. homogeneously mixing 1 al. acicular alphaFe O of high surface area,

V b. BaGO or equivalent source of barium oxide in amount to provide uponreaction with the alpha- Fe O a ferrite of the generalized formula BaFec. about 1-6 percent of NaF, fluxing agent, and d. a lead compound in anamount exceeding 4 weight percent and less than 1% weight percent of themixture, 2. calcining the mixture at about 850-l 100C, and 3. treatingthe ferrite with aqueous acid solution to remove undesirable reactionproducts and any unreacted material to provide elongated platelets. 3.Process comprising the steps of: l. homogeneously mixing materials whichare reasonably free from impurities and comprise a. acicular alpha-Fe Ohaving a surface area of at least 15 square meters per gram,

b. BaCO in amount to provide upon reaction with the alpha-Fe- O aferrite of the generalized formula BaFe O c. 5-12 weight percent of NaF,and d. PhD in an amount exceeding weight percent and less than 1% weightpercent of the mixture, 2. calcining the mixture at about 850l 100C, 3.treating the ferrite with aqueous acid solution to remove undesirablereaction products and any unreacted material to provide elongatedplatelets which are especially adapted to mechanical orientation inadmixture with a workable nonmagnetic matrix material,

4. mixing the elongated platelets with a workable rubber matrix materialto provide a mixture comprising about 55-70% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets bymechanical shearing forces, and

6. then vulcanizing the matrix material to provide a permanent magnethaving a maximum energy product greater than 1.5 X l gauss-oersteds.

4. Process comprising the steps of:

l. homogeneously mixing a. acicular alpha-Fe O particles having asurface area of at leat 15 square meters per gram,

b. BaCO or equivalent source of barium oxide in amount to provide uponreaction with the alpha- Fe O particles a ferrite of the generalizedformula BaFe O c. about l-l2 weight percent of NaF fluxing agent,

and

d. where the amount of fluxing agent in weight percent x is less than 6,at least weight percent of a lead compound up to about 2 weight percentof the mixture,

2. calcining the mixture at about 850l 100C,

3. treating the ferrite with aqueous acid solution to remove undesirablereaction products and any unreacted material to provide elongatedplatelets,

4. mixing the elongated platelets with a workable rubber orthermoplastic matrix material to provide a mixture comprising about5570% ferrite by volume,

5. rolling or extruding the mixture to align the ferrite platelets bymechanical shearing forces, and

6. then vulcanizing the rubber or cooling the thermoplastic matrixmaterial to lock the ferrite platelets in place to provide permanentmagnet material having a maximum energy product of at least 1.4 Xgauss-oersteds.

5. In an process comprising the steps of:

l. homogeneously mixing alpha-Fe O particles with BaCO or equivalentsource of barium oxide in amount to provide upon reaction with thealpha- Fe O particles 21 ferrite of the generalized formula IZ ISv 2.calcining the mixture to provide barium ferrite platelets,

3. mixing the ferrite platelets with a workable rubber or thermoplasticmatrix material to provide a mixture comprising about 5570% ferrite byvolume,

4. rolling or extruding the mixture to align the ferrite platelets bymechanical shearing forces, and

5. then vulcanizing the rubber or cooling the thermoplastic matrixmaterial to lock the ferrite platelets in place to provide permanentmagnet material,

the improvement comprising:

a. step 1) employs acicular 'alpha-Fe O particles having a surface areaof at least 15 square meters per gram,

b. included in step (1) is about 1l2 weight percent of NaF fluxing agentand where the amount of fluxing agent in weight percent x is less than6, at least weight percent of a lead compound up to about 2 weightpercent of the mixture, c. the calcining step (2) is 850-110() c, (1.step (2) is followed by treating the ferrite with aqueous acid solutionto remove undesirable reaction products and any unreacted material toprovide elongated platelets. 6. .Process as defined in claim 5 whereinPbO is added carried out at in step (1) in an amount exceeding 4 weightpercent.

present in an amount exceeding weight percent.

1. PROCESS COMPRISING THE STEPS OF,
 1. HOMOGENEOUSLY MIXING A. ACICULARALPHA-FC2O3 OF HIGH SURFACE AREA, B. BACO3 OR EQUIVALENT SOURCE OFBARIUM OXIDE IN AMOUNT TO PROVIDE UPON REACTION WITH THE ALPHA-FE2O3 AFERRITE OF THE GENERALIZED FORMULA BAFE12O19, C. ABOUT 1-12 WEIGHTPERCENT OF NAF FLUXING AGENT, AND D. WHERE THE AMOUNT OF FLUXING AGENTIN WEIGHT PERCENT "X" IS LESS THAN 6, AT LEAST
 2. CALCINING THE MIXTUREAT ABOUT 850*-1100*C, AND
 2. Process for making barium ferrite particleswhich are especially adapted to mechanical orientation in admixture witha workable nonmagnetic matrix material, which process comprises thesteps of:
 2. calcining the mixture at about 850*-1100*C, and 2.calcining the mixture at about 850*-1100*C,
 2. calcining the mixture atabout 850*-1100*C,
 2. calcining the mixture to provide barium ferriteplatelets,
 2. calcining the mixture at about 850*-1100*C, and 3.treating the ferrite with aqueous acid solution to remove undesirablereaction products and any unreacted material to provide elongatedplatelets which are especially adapted to mechanical orientation inadmixture with a workable nonmagnetic matrix material.
 3. mixing theferrite platelets with a workable rubber or thermoplastic matrixmaterial to provide a mixture comprising about 55-70% ferrite by volume,3. treating the ferrite with aqueous acid solution to remove undesirablereaction products and any unreacted material to provide elongatedplatelets,
 3. treating the ferrite with aqueous acid solution to removeundesirable reaction products and any unreacted material to provideelongated platelets which are especially adapted to mechanicalorientation in admixture with a workable nonmagnetic matrix material, 3.treating the ferrite with aqueous acid solution to remove undesirablereaction products and any unreacted material to provide elongatedplatelets.
 3. Process comprising the steps of:
 3. TREATING THE FERRITEWITH AQUEOUS ACID SOLUTION TO REMOVE UNDESIRABLE REACTION PRODUCTS ANDANY UNCREATED MATERIAL TO PROVIDE ELONGATED PLATELETS WHICH AREESPECIALLY ADAPTED TO MECHANICAL ORIENTATION IN ADMIXTURE WITH AWORKABLE NONMAGNETIC MATRIX MATERIAL.
 4. rolling or extruding themixture to align the ferrite platelets by mechanical shearing forces,and
 4. mixing the elongated platelets with a workable rubber matrixmaterial to provide a mixture comprising about 55-70% ferrite by volume,4. mixing the elongated platelets with a workable rubber orthermoplastic matrix material to provide a mixture comprising about55-70% ferrite by volume,
 4. Process comprising the steps of:
 5. In anprocess comprising the steps of:
 5. then vulcanizing the rubber orcooling the thermoplastic matrix material to lock the ferrite plateletsin place to provide permanent magnet material, the improvementcomprising: a. step (1) employs acicular alpha-Fe2O3 particles having asurface area of at least 15 square meters per gram, b. included in step(1) is about 1-12 weight percent of NaF fluxing agent and where theamount of fluxing agent in weight percent ''''x'''' is less than 6, atleast
 5. rolling or extruding the mixture to align the ferrite plateletsby mechanical shearing forces, and
 5. rolling or extruding the mixtureto align the ferrite platelets by mechanical shearing forces, and 6.then vulcanizing the matrix material to provide a permanent magnethaving a maximum energy product greater than 1.5 X 106 gauss-oersteds.6. then vulcanizing the rubber or cooling the thermoplastic matrixmaterial to lock the ferrite platelets in place to provide permanentmagnet material having a maximum energy product of at least 1.4 X 106gauss-oersteds.
 6. Process as defined in claim 5 wherein PbO is added instep (1) in an amount exceeding 3/4 weight percent.
 7. Process asdefined in claim 4 wherein a magnetic field is employed as part of step(5) to supplement the mechanical shearing forces in order to improve thealignment of the ferrite platelets.
 8. Process as defined in claim 1wherein the fluxing agent is in the amount of 3-10 weight percent. 9.Process as defined in claim 4 wherein the fluxing agent is in the amountof 3-10 weight percent.
 10. Process as defined in claim 1 wherein PbO ispresent in an amount exceeding 3/4 weight percent.